1. Field of the Invention
The present invention relates to touch panels adhered on surfaces of display devices, such as liquid crystal displays (LCDs). More particularly, the present invention relates to a method for driving a touch panel with small electric power and an apparatus for executing the method.
2. Description of the Related Art
For example, an input operation is performed in a following manner with transparent touch panels adhered on various display screens. The screens display, for example, various images associated with instructions to be input. Users touch one of the images with their finger. A detecting unit of the touch panels according to various technologies then detects the touched position of the screens in an X-Y plane. The detecting unit identifies an instruction input by the users by determining the instruction associated with the image displayed at the detected position. At this time, the users can specify not only a point but also a line and an area by continuously tracing the points and can display the line and the area.
Although various technologies exist for such touch panels, resistive touch panels are widely used because they can offer excellent position detection accuracy, stable operations, few troubles, and a relatively low manufacture cost. Various methods exist for the resistive touch panels. Many of the methods have a cross section illustrated in FIG. 12A. More specifically, sealing members 2 for sticking upper and lower panels together and dot spacers 3 for preventing short-circuit of upper and lower electrodes are arranged on a surface of a base glass substrate 1. A resilient sheet 4 made of glass or polyethylene terephthalate (PET) is adhered on surfaces of the sealing members 2. In this way, a touch panel 5 is formed. The sealing members 2 have a thickness of approximately 5-10 μm, whereas the sheet 4 has a thickness of approximately 200 μm. A transparent fixed-side resistive film 6 of indium tin oxide (ITO) is disposed on a surface of the glass substrate 1 that faces the sheet 4. A transparent movable-side resistive film 7 of ITO is disposed on a surface of the sheet 4 that faces the glass substrate 1. The resistive films 6 and 7 have electrodes on opposing sides thereof to receive voltage applied thereto.
A four-wire configuration is a basic electrode configuration. FIG. 12B illustrates a method for detecting a position of a point touched with a finger in the four-wire configuration. In an example illustrated in FIG. 12B, to detect a touched position in a Y-axis direction of a X-Y plane, fixed-side electrodes Y1 and Y2 are disposed on opposing horizontal sides of a fixed-side resistive film. Voltage is applied to the electrodes Y1 and Y2 so that the electrodes Y1 and Y2 function as positive and negative electrodes, respectively. To detect the touched position in an X-axis direction, movable-side electrodes X1 and X2 are disposed on opposing vertical sides of a movable-side resistive film. Voltage is applied to the electrodes X1 and X2 so that the electrodes X1 and X2 function as positive and negative electrodes, respectively. The fixed-side electrodes and the movable-side electrodes face each other through spacers (not illustrated).
If a user touches a given point P of a touch panel having such an electrode configuration, the movable-side resistive film bends in the direction of the spacers to come into contact with the fixed-side resistive film. In response to the contact, resistances for dividing the applied voltage are produced in the movable-side and fixed-side resistive films. Resistance values of these resistances can be determined. More specifically, in the illustrated example, resistances Rx1 and Rx2 are produced between the electrode X1 and the point P and between the electrode X2 and the point P, respectively. Similarly, resistances Ry1 and Ry2 are produced between the electrode Y1 and the point P and between the electrode Y2 and the point P, respectively.
As illustrated in FIG. 13, the positions of the point Pin the X-axis and Y-axis directions can be detected by measuring, with a voltmeter V, with connections being switched by a resistance-measuring switch SW, voltage values affected by the resistances Rx1 and Rx2 and the resistances Ry1 and Ry2 produced in response to the contact of the X-side and Y-side resistive films at the point P, respectively. In the illustrated example, the positions of the point P in the X-axis and Y-axis directions are detected as Ex and Ey, respectively, by operating the switch SW.
As described above, a touch panel includes touch detecting surfaces functioning as resistive surfaces for detecting resistances, electrodes, a power system for the electrodes, and lead circuits to an exit portion connected to a flexible printed circuit board (FPC) having circuits for signals for use in measurement of the resistance values. FIG. 14 illustrates a glass substrate 1 having fixed-side electrodes 7 an 8 of a touch panel. For example, the electrodes 7 and 8 disposed on respective sides of a fixed-side resistive film 6 are connected to a lead-circuit exit portion 11 through lead circuits. Similarly, electrodes of a movable-side resistive film (not illustrated) are connected to the lead-circuit exit portion 11 through lead circuits 9 and 10. The arrangement of the lead circuits of the resistive films often differs for each product.
In the method illustrated in FIG. 13, the point touched by a user on the touch panel is detected with connections being switched by the switch SW. In practice, the switching operation is performed in a manner illustrated in FIG. 10, for example. More specifically, in an example illustrated in FIG. 10, an electronic circuit alternately applies voltage to the opposing electrodes of the movable-side resistive film (hereinafter, referred to as X-side electrodes) and the opposing electrodes of the fixed-side resistive film (hereinafter, referred to as Y-side electrodes) every 10 milliseconds (ms) to demonstrate the switching function. Here, the switching interval is set to 10 ms because of the following reason. Many currently used touch panels measure resistance values using a fine pulse of 2.5-3 ms as illustrated in FIG. 11D, for example. Upon successively acquiring the same data twice, the touch panels output the data as the measured resistance value.
As illustrated in FIG. 11A, the first 3 ms of a touch operation, which can also be referred to as a switching operation caused by the contact of the resistive films in response to the user touch operation on the touch panel, is in a chattering state where the contact is unstable just like various other switching operations. An accurate resistance value is not available with the pulse for detection of the resistance value generated during this period. When pulses generated every 3 ms are used to detect coordinates of the touched position, it takes 6 ms or more to successively acquire the same result from measurement of the resistance value with the following two pulses. When the measurement is performed every 2.5 ms, it takes 5 ms or more. Here, as illustrated in FIGS. 11A-11D, it is assumed that the switching operation is performed every 10 ms and a touch operation is performed during a period when measurement on the X side is enabled. In such a case, measurement executed during the first 2.5-3 ms yields an inaccurate value but measurement can be executed twice or more during the remaining period, i.e., approximately 7 ms.
In consideration of such respects, many touch panels switch power distribution every 10 ms so that voltage is alternately supplied to the X-side electrodes and the Y-side electrodes. The switching operation performed every 10 ms is effective to detect a touch operation as rapidly as possible. However, when the touch panel receives a manual input operation, such as one performed with a finger, the input speed has a limit. Even an expert of fast pushing of buttons of a game platform can perform the input operation 16 times per second. When the input operation is performed 25 times per second, an interval of 40 ms is long enough to detect the input operation. However, in many cases, the interval is set to 10 ms in consideration of various usage states of the touch panels described above.
A discussion will now be given for a total amount of current consumed by a touch panel illustrated in FIGS. 11A-11D according to the related art when the power distribution to the X side and the Y side is switched every 10 ms. FIGS. 15A and 15B illustrate results obtained by actually measuring resistance values of currently used touch panels. More specifically, FIG. 15A illustrates a result obtained from measurement on 130 touch panels of the same type for a 4.3-inch widescreen monitor having an aspect ratio of 16:9. The result indicates that minimum, maximum, and average resistance values on the X side (horizontal direction) of the touch panels were 387 Ω, 607Ω, and 470Ω, respectively. In contrast, minimum, maximum, and average resistance values on the Y side were 275 Ω, 413Ω, and 331Ω, respectively. FIG. 15B illustrates a result obtained from measurement on 42 touch panels of the same type for an 8.4-inch widescreen monitor having an aspect ratio of 4:3. In the touch panels of this type, minimum, maximum, and average resistance values on the X side were 544Ω, 648Ω, and 593Ω, respectively. In contrast, minimum, maximum, and average resistance values on the Y side were 351Ω, 375Ω, and 362Ω, respectively.
Even the touch panels of the same type have the varying resistance values because of variation of surface resistivity of ITO resistive films, variation of thickness of the ITO resistive films, variation of silver electrodes disposed on the respective sides of the resistive films, and variation of length of a silver-electrode circuit, including the electrodes and lead circuits illustrated in FIG. 14, used in measurement of the resistance values.
The resistance-value variation is discussed about, for example, the 4.3-inch touch panels having the aspect ratio of 16:9 illustrated in FIG. 15A. A touch panel may exist that have the X-side and Y-side resistance values of 607Ω and 275Ω, respectively, when a difference between the X-side and Y-side resistance values is the largest. An amount of current consumed by the touch panel having such resistance values in a standby state is discussed. As illustrated in FIG. 10, when a voltage of 3.3 V is commonly applied to the X-side and Y-side electrodes in the related art, a current Ix of the X-side resistive film is determined based on the commonly applied voltage Vc and the resistance value of 607Ω of the X-side resistive film selected in the above-described manner. More specifically, the current Ix is determined as follows: Ix=(commonly applied voltage Vc)/(X-side resistance value Rx)=3.3 V/607 Ω=0.0054366 A. That is, a current of 0.0054366 A (=5.436 mA) flows through the X-side resistive film per second. Accordingly, a current of 0.0054366 mA flows per millisecond.
Similarly, an amount of current consumed by the Y-side resistive film is determined. Since the voltage commonly applied to the X-side and the Y-side resistive films is equal to 3.3 V and the resistance value selected in the above-described manner is equal to 275Ω, a current Iy of the Y-side resistive film is determined as follows: Iy=Vc/Ry=3.3 V/275Ω=0.01200 A. Accordingly, a current of 0.01200 mA flows per millisecond. Since voltage is alternately applied to the X-side and Y-side resistive films of this touch panel every 10 ms, an amount current consumed during 100 ms is determined to cope with an actual usage state of the touch panel. A 10-ms pulse is supplied to the X-side resistive film five (=100 ms/(10+10) ms) times. Accordingly, as illustrated in FIG. 10, an amount of current consumed by the X-side resistive film during 100 ms is calculated as follows: (Ix)×10 ms×(100/20) times=0.2718 mA/100 ms. Similarly, an amount of current consumed by the Y-side resistive film during 100 ms is equal to 0.6000 mA/100 ms. Accordingly, a sum of the amounts of current consumed by the x-side and Y-side resistive films during 100 ms is equal to 0.8718 mA/100 ms.
Japanese Unexamined Patent Application Publication No. 9-152932 discloses a technology for decreasing power consumption of a touch panel by extending intervals at which driving current is intermittently applied to the touch panel if it is determined that an input operation on the touch panel is not continued.